Growth and Physiology of Artichoke Transplants Exposed to ABA, Heat and Drought Stresses

In southwest Texas, artichoke (Cynara scolymus L. cv. Green Globe Improved) seedlings planted in the field late summer or early fall generally experience simultaneous high air temperatures and short drought episodes, causing transplant shock and delay in root and shoot development. A growth chamber study was conducted to determine the effects of heat (35/20^oC, day/night temperature regimes vs. 25/10^oC) and drought (daily maintenance at 30% water holding capacity vs. 60%) stresses on growth, physiology and water status of artichoke transplants during 14 days after transplanting (DAT). Heat stress significantly inhibited shoot but not root growth 14 DAT. In contrast, drought stress significantly inhibited root growth between 7 and 14 DAT. Plant physiological parameters including transpiration (E), stomatal conductance (g_s) and photosynthetic rate (Aco₂) decreased by transplanting in all treatments (≤24h). Thereafter, E and Aco₂ significantly increased by heat compared to 25/10^oC temperature regimes 3 DAT. However as heat stress progressed there was a reduction in E, g_s Aco₂ and leaf water potential. Drought stress sharply reduced E, g_s and Aco₂ within 1 DAT and leaf water potential and relative water content as early as 3 DAT. The combination of heat and drought stress significantly reduced shoot and root growth 3 DAT and all physiological parameters within 1 DAT, and plants did not recover throughout the 14 day-period. This suggests that to minimize transplant shock by heat and drought it is desirable to either condition seedlings to improve early root growth and/or to reduce leaf water loss by transpiration. A follow up greenhouse study evaluated the effects of three film-forming antitranspirants (Antistress, Transfilm and Vapor Gard) and abscisic acid (ABA, 500-2000 mg·L^-1) foliar application on physiological responses and shoot water status of artichoke transplants exposed to drought stress (4-day water withholding). ABA at 1000 mg·L^-1 enhanced drought tolerance of transplants, a response that was associated with the maintenance of shoot water status via stomatal closure, whereas film-forming antitranspirants were not effective to mitigate drought stress. These results suggest that exogenous ABA could be a useful plant growth regulator to condition artichoke transplants to withstand temporal drought stress conditions and reduce transplant shock.